Fast self-quenching of dynamic scattering in liquid crystal devices

ABSTRACT

A liquid crystal element is operated within a critical frequency range such that upon removal of the exciting electric field, the dynamic scattering exhibited by the crystal decays within a matter of milliseconds- an interval which is substantially shorter than the usual natural decay time. The critical frequency range is related to the conductivity (which is temperature and voltage dependent) and dielectric constant of the liquid crystal and is precisely defined by equations in the detailed description below.

United States Patent George Harry IIeilmeier Philadelphia, Pa. 847,659

Aug. 5, I969 Apr. 20, 1971 RCA Corporation [72] Inventor [2 l Appl. No. [22] Filed [45] Patented [73] Assignee [54] FAST SELI QUENCIIING OF DYNAMIC SCATTERING IN LIQUID CRYSTAL DEVICES 2 Claims, 3 Drawing Figs.

[52] U.S,Cl 350/160 [51] Int. Cl G02f 1/28 [50] Field of Search 350/160,

(LC. Digest), (S. Lib.)

[56] References Cited UNITED STATES PATENTS 3,519,330 7/ I970 Heilmeier Primary Examiner-Ronald L. Wibert Assistant Examiner-J. Rothenberg Attorney-H. Christofi'ersen A.C. SOURCE D =ZZ LIQUID CRYSTAL FREE SPACE WHERE= 0' CONDUCTIVITY OF LIQUID CRYSTAL Z DIELECTRIC CONSTANT OF Z DIELECTRIC CONSTANT OF LIQUID CRY'SJAL PATENIEI] APRZU IEIII AC. SOURCE Z DIELECTRIC CONSTANT OF LIQUID CRYSTAL Z DIELECTRIC CONSTANT OF CINYQSJNDIL I0 FREE SPACE Fig. 1.

LIGHT SCATTERING INTENSITY Fig. 2.

APPLIED VOLTAGE 50 VOLTS/ DIV.

I000: O TIME 5MS/DlV. 2 -5 2 g 200- S E IOOE x g Fig. 3. u APPLIED VOLTAGE=85 V.rms g CELL THICKNESS=O.5 MILS f 3 Geore H. HeI'Ime/er IO I By 2 5 I0 20 50 L P=RESISTIVITY IN III- X10 4 A T TOR/IE) FAST fiElLF-QUENCG F DYNAMIC SCATTERING IN lLllQll CRYSTAL DEVICES BACKGROUND OF THE INVENTION The article Dynamic Scattering: A New Electra-optic Effect" by G. H. Heilmeier, L. A. Zanoni and L. A. Barton, Proceedings of the IEEE, Volume 56, No. 7, July, 1968, discusses nematic liquid crystals of a type of interest in the present application. As explained there, such crystals, when in an unexcited state, are relatively transparent to light and when placed in an excited state by an applied electric field, scatter light. The light scattering, termed dynamic scattering, is believed to result from turbulence developed in the crystal by ions in transit.

The dynamic scattering exhibited by liquid crystals may be employed in reflective, absorptive and transmission type flat ...panel displays, in light shutters and in other applications.

However, the natural recovery time of the crystal, that is, its tumoff time, which is a function of such factors as the crystal material, the spacing between electrodes, and the crystal temperature, is relatively long. For example the tumoff time at room temperature for typical materials may be from 30 to several hundred milliseconds (l millisecond 0.00] second). For certain applications, the long turnoff time is a serious disadvantage. For example, when the information signal of a liquid crystal display system rapidly change its value, the liquid crystal elements of the display matrix may not as rapidly be able to follow these changes. The result is objectionable smear of moving objects, known as trailing edge smear.

The turnoff time of the liquid crystal can be reduced by applying thereto a quenching electrical signal. This signal can be either a short, direct voltage pulse, that is, a pulse of sufficient amplitude to align the dipoles of the liquid crystal but of insufficient duration to cause substantial ion injection into the liquid crystal or it can be a relatively high frequency alternating voltage signal. The frequency of the latter, it has introduce found, must be greater than that which will cause dynamic scattering to occur and lower than some upper frequency limit. For example, for some crystals, an alternating voltage quenching electrical signal in the range of 2 to 20 kilohertz has been found substantially to reduce the liquid crystal turnoff time. in both the case of the direct voltage and alternating voltage quenching signal, the electric field due to the signal snap orients the dipoles of the turbulent fluid without introducing additional disruption due to ions in transit. In other words, the applied fields introduce a dielectric torque which is much larger than the conduction induced torque.

While the above turnoff methods are perfectly suitable, they do require additional circuits and this increases the expense of the liquid crystal display or other system. The object of the present invention is to provide a method for operating a liquid crystal in such a way that it naturally self-quenches at a speed comparable to that obtained previously by the methods discussed above but without the need for a turnoff circuit.

SUMMARY OF THE INVENTION e is the dielectric constant of the liquid crystal material; and

e,,=8.85 l 0 farads/centimeter and is the dielectric constant of free space.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagram of a circuit for practicing the method of the present invention;

FIG. 2 is a drawing of waveforms to explain the operation of the circuit of HG. l; and

FIG. 3 is a graph showing the lower cutoff frequency for self-quenching as a function of the resistivity of the liquid crystal.

DETAILED DESCRIPTION The liquid crystal element shown schematically at 10 in FIG. 1 is basically a parallel plate capacitor with a liquid crystal dielectric. The method of fabricating the device has been described previously in the literature as, for example, in G. H. Heilmeier, L. A. Zanoni and L. A. Barton, Applied Physics Letters 13, 46 (I968). The plates are conductors at least one of which is transparent and they are usually formed on glass. The thickness of the liquid crystal layer between the plates may be of the order of 25 microns or less, that is, 1 mil or less and is held in place between the plates by capillary action.

In operating a liquid crystal element such as this, the present inventor has discovered that by controlling certain of the liquid crystal cell parameters and the operating frequency and amplitude of the alternating voltage employed to produce the dynamic scattering, that upon the removal of the voltage, the liquid crystal self-quenches in a very short interval of time. For example, in the circuit shown in FIG. I in which the liquid crystal has a material dielectric constant e of about 3.5 and a conductivity of about 10" (ohm-centimeters) and in which the frequency applied by source 12 is a gated 600 hertz sine wave, the performance shown in FIG. 2 is obtained.

The lower waveform of Fig. 2 is the voltage applied to the liquid crystal. The upper waveform of Fig. 2 consists of two parts. The oscillations within envelope 14 are obtained with the aid of a photomultiplier positioned to receive the light reflected from the liquid crystal 12 when it is excited by the lower wave of FIG. 2. The photomultiplier has a relatively short time constant and is able to follow the variations in intensity of the actual light scattering which is produced. Each time the applied voltage causes maximum ion motion, and this occurs twice each cycle, the light scattering is maximum, and between these intervals the light scattering is minimum. This explains why the frequency of the light scattering wave is double that of the applied voltage wave. The heavy line 14, which is the envelope of the light scattering curve, is what is observed by the eye. The eye intergrates the amount of light scattered by the light crystal, as it is not able to follow the rapid change from maximum to minimum light scattering condition of the liquid crystal. This curve 14, for the major portion of its extent, is similar to what would be obtained by applying a direct voltage pulse to the liquid crystal or by applying a conventional alternating voltage tum-on pulse to the crystal, both as described in the introductory portion of the present application. However, with the system parameters adjusted in the manner discussed in detail below, when the alternating voltage produced by source 12 is terminated, the light scattering intensity very abruptly decays. The decay time, that is, the turnoff time, is roughly 3 milliseconds. The same liquid crystal under direct voltage excitation has a turnoff time of approximately milliseconds! The present inventor has found experimentally that a liquid crystal which rapidly self-quenches after alternating voltage excitation exhibits the following properties:

I. For a given amplitude of the applied alternating voltage, there is a critical frequency range over which fast selfquenching is observed. Below this range, the decay time increases markedly while above it no dynamic scattering is observed.

2. Raising the amplitude of the applied alternating voltage while maintaining the frequency constant flat the decay time. Fast self-quenching can be restored if the frequency natural also raised. As will be shown shortly, according to the theory of operation which has been formulated, this effect can be explained in terms of the conductivity of the liquid crystal, the conductivity being voltage dependent.

3. Raising the temperature of the liquid crystal while maintaining the amplitude of the applied voltage constant requires an increase in the frequency of the voltage to retain the fast self-quenching characteristic of the liquid crystal. Again, raising the temperature raises the conductivity and it is the conductivity which is one of the factors upon which the Substituting equation (l) into equation (3) and solving for f gives:

fast self-quenching effect depends. 5

4. As the conductivity of the material is increased, the The last equation above takes into account the four frequency range of the applied voltage necessary to permit observed characters given previously. The conductivity is a fast self-q n hing is shifted to a h g ge- I I116 function of the applied electric field and this, in turn, is a conductivity of the liquid crystal is made too high, how function of the amplitude of the alternating voltage driving fast self'quenchmg 15 longfir obselved- I I 10 signal. The conductivity also is a function of temperature. The

It has been observed that when operating in the fast se frequency f is directly proportional to the conductivity and q g mode essemlally 100 Percent modulatlon 0f the inversely proportional to the dielectric constant e of the liquid scattered hght is obtained. In other words, the llqllltl crystal crystal,

Produces substantially the maximum amount of ligh As mentioned above, for typical liquid crystal material 6 scattering which is possible. This beha i r i Obtained, l0 (ohms ems) and e 3.5. Substituting these numbers into however, only when the current through the liquid crystal cell equation pt and solving gives: i i i 77 leads the" voltage by 55 to 60 or more. As the lead angle 1 4 10-10 between current and voltage decreases to less than 55, f m HZ- modulation decreases sharply from 100 percent.

The clues above have led to the development of the This value is approximately what is observed experimentally following theory as to why selfquenching occurs and to a for excitation voltages below 40 volts. At higher voltages, the determination of the conditions which are necessary for selfconductivity increases with voltage and hence the lowest quenching. The tangent of the phase angle I discussed above frequency at which self-quenching can occur increases. is simply the ratio of displacement current I to the A plot of the experimentally determined lower cutoff conduction current I where the conduction current refers to frequency for self-quenching as a function of material the flow of free charges and the displacement current is resistivity for a driving voltage amplitude of 85 volts RMS, is analogous to the current employed to charge acapacitor. given in FIG. 3. It is clear that the cutoff frequency varies inversely with resistivity as predicted by equation (4). Once t above the critical frequency, self-quenching occurs up to the 10 cutoff frequency for dynamic scattering. This value is Where determined by space charge buildup and is roughly equal to E =electric field the reciprocal of the dielectric relaxation time, that is, it is o'= the field (and temperature) dependent conductivity of roughly equal to 20/0.

the liquid crystal The effect discussed herein is present in nematic liquid D=ee,,, where crystals of the type operating on the principle of dynamic s the free space dielectric constant scattering. Some typical examples are both the compounds of 8.85Xl0 farads/centimeter I Table 1 below and the mixtures of Table II below. in Table H,

e the dielectric constant of the liquid crystal and for a mixture 11 is typical material may be of the order of 3.5 40

w frequency in radians 211], where 0 f frequency cHio oH=N- -0 ii CH: and

Experimentally, as indicated above, self-quenching requires d) 55. Therefore:

tan 1 1.4 (2) I l .4 I (3) has transition temperatures of 81C. and 1 10 C.

TABLE I.(COMPOUNDS) Crystal- Mesomorphicmesomorphie isotropic Compound transition liquid transi- Example X Y temp., 0. tion temp., C

1 cHacHzi l OCH3 86 118 i 2 CH3(CH2)2C 0- Same as above 86 119 3 flame as above 0 CeHra 86 120 if 2 C4Ha0 -OCCH3 82 113 5 iso C5H11O- Same as above. 74 82 is C H O as 109 i CsH11O so 105.5 E mmo- 86 100 ll 9 CHaO- OC(CH2)2CH3 i 113 1 0 CH O Same as above TABLE II.(.\IIXTURES) Ciystal- Mesomorphlcmesomorphic isotropic Mixture transition liquid transi- Example Components temp, C. tion temp., C.

A 50% 1 150%5 47 108 B 35.1% 1 t32.6%1-32.3% 40 103 50% g. D 25% each of ii, 4 ti and 39 104 30.6% G wt. ratio each of 22 105 2, an H $6 wt. ratio each of 25 105 Q I 60% 240% 1 0 r 45 106 The voltage range over which the effect has been observed extends from roughly 40 to over 200 volts. However, the particular voltage range for a particular sample is expressed in the equation already given, as 0' the conductivity, is voltage dependent. So, for a given frequency and dielectric constant, and initial value of a, the voltage range readily may be determined and for a given voltage, given initial value of oand given dielectric constant, the frequency range may be determined, the lower limit of the latter being shown in FIG. 3.

Following is a qualitative explanation of why the selfquenching discussed above occurs. In the usual operation of a liquid crystal in which an altemating-frequency field is employed to produce dynamic scattering, there are two competing effects. One is the tendency of the dipoles of the crystal (the rodlike elements) to follow the field. When the field is in one direction, the dipoles tend to line up with the field in that direction and as the field changes its direction, the dipoles tend to flip over and align with the field in the other direction. This effect tends to cause the liquid crystal to look relatively transparent because when the dipoles become aligned in a regular pattern there is very little light scattering from the dipoles. The second effect is that the voltage creating the electric field causes ions to be injected into the liquid crystal and these ions tend to cause turbulent motion of the dipoles making up the crystal. The turbulent motion manifests itself as the dynamic scattering of light. In other words, this competing effect tends to make the crystal relatively opaque.

In the normal alternating voltage operation of the crystal, when the alternating electric field is removed, the crystals are in relatively disorientated state and it takes a considerable time-the 30 to several hundred milliseconds mentioned in the introductory portion of this applicationfor the dipolar regions to naturally relax and to look relatively transparent. In the method of operation of the present application, the same two competing effects as discussed above are present. However, when the circuit and device parameters are those given, while during the application of the alternating voltage to the crystal the domains of the liquid crystal are sufficiently disorientated relative to one another to produce dynamic scattering, they are not as disorientated as in the usual method of operation. Each small domain within the crystal, each of which domain consists of aligned dipoles, attempts to follow the relatively high driving electric field as the field changes its polarity. However, the injected ions disturb the domains sufficiently that each such domain is slightly out-of-phase with its adjacent neighbor domains. While the efiect observed by the eye is that of dynamic scattering just as in the usual manner of operation, when the alternating field is removed, there is much less reorientation which is required to place the various domains into alignment again. The liquid crystal fluid, in other words, is in a more uniform state than in the case of pure dynamic scattering. As a result, the scattering returns rapidly (in a matter of under 10 milliseconds and generally is less than 5 milliseconds) to its residual valuethe value at which it appears to the eye to be transparent.

lclaim:

l. A method of operating a nematic liquid crystal of the type which exhibits dynamic scattering comprising the step of applying to the crystal an alternating voltage at a frequency F greater than approximately l.4o-/21rD and not more than the upper limit at which dynamic scattering occurs and of sufficient amplitude to cause dynamic scattering, whereby upon removal of said voltage the liquid crystal returns to its unexcited condition in a relatively short interval of time, where a is the conductivity in (ohm-centimeters) of the liquid crystal, D=ee,,, e is the dielectric constant of the liquid crystal, and e,,=8.85X10" farads/centimeter the dielectric constant of free space.

2. A method of operating a nematic liquid crystal of the type which exhibits dynamic scattering to insure self-quenching within a short interval after the exciting voltage has been turned off comprising, the step of applying to the crystal an alternating voltage of sufi'lcient amplitude to cause dynamic scattering and in the frequency range between the limits of about l.4cr/21rbD and about 2o/D, where: ois the conductivity in (ohm-centimeters) D=ee,,, e is the dielectric constant of the liquid crystal, and e,,=8.85 l0" farads/centimeter and is the dielectric constant of free space.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 D t d April 20 1971 Inventor(s) George Harry Heilmeier It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 65, after insert ---the--- Column 1 line 65, "(ohm-centimeters) should read 2ms-centimeters)' Column 1, line 68 "10 should read ---l0' Column 2, line 26, "10 ohm-centimeters) should read --l0" (ohm-centimeters) Column 3 line 38 "10 should read ---l0' Column 4, line 15, "lo g sohms cms) should read (ohms cms)' Table I last line should read -OC(CH CH Column 6, line 36, "(ohm-centimeters) should read Column 6, line 38 "l0 --iozm-centimetersy'lshould read --l0' Column 6 line 47, (ohm-centimeters) should read o m-centimeters)' Column 6 line 48 "10 1 sho ld r d 10" Signed and sealed this 21 at day of December 1 971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOT'ISGHALK Attesting Officer Acting Commissionerof Patents FORM PC3-1050 (10-69) USCOMM-DC e03: 

2. A method of operating a nematic liquid crystal of the type which exhibits dynamic scattering to insure self-quenching within a short interval after the exciting voltage has been turned off comprising, the step of applying to the crystal an alternating voltage of sufficient amplitude to cause dynamic scattering and in the frequency range between the limits of about 1.4 sigma /2 pi D and about 2 sigma /D, where: sigma is the conductivity in (ohm-centimeters) 1, D epsilon epsilon o, epsilon is the dielectric constant of the liquid crystal, and epsilon o 8.85 X 10 14 farads/centimeter and is the dielectric constant of free space. 